1. Field of the Invention
This invention relates generally to an organic electronic device, in particular, to an electrode that is substantially transparent and conductive and incorporated in the organic electronic device.
2. Description of the Related Art
Organic electronic devices include those which convert electrical energy into optical energy, or vice versa, as well as those that detect optical signals through electronic processes. Such organic electronic devices include OLEDs, solar cells, phototransistors, photodetectors, lasers, and opto-couplers. Such devices typically include a pair of electrodes (e.g., an anode and a cathode) with at least one charge-carrying layer between the electrodes. Depending on the function of the device, the charge-carrying layer or layers may be comprised of a material or materials that are electroluminescent when a voltage is applied across the device or the layer or layers may form a heterojunction capable of generating a photovoltaic effect when exposed to optical radiation.
In the particular case of the OLED, the OLED is typically comprised of two or more thin organic layers (e.g., a conducting polymer layer and an emissive polymer layer where the emissive polymer layer emits light) separating its anode and cathode. Under an applied potential, the anode injects holes into the conducting polymer layer, while the cathode injects electrons into the emissive polymer layer. The injected holes and electrons each migrate toward the oppositely charged electrode and produce an electroluminescent emission upon recombination in the emissive polymer layer.
The material that is used as the cathode layer of the OLED is typically multilayer and comprised of generally a thin electron injecting layer that has a low work function and also a thick conductive layer such as aluminium or silver. The electron injecting layer provides an electrically conductive path for current flow as well as a way to efficiently inject electrons into the adjacent emissive polymer layer. The conductive layer has to be thick enough to be adequately conductive, however, the thickness providing adequate conductivity results in the cathode layer being highly reflective. Transparent electrode materials such as indium tin oxide (“ITO”) cannot be used as a cathode because it is typically deposited in a manner that causes damage to the organic layer within the OLED and also because it does not have a low work function.
For the foregoing reasons, there exists a need for a cathode that is conductive, substantially transparent, has a low work function, and can be deposited in a manner that doesn't damage the organic layers of the organic electronic device.
In an OLED display, it is difficult to achieve reasonable contrast of the image generated by the display when ambient light emitted from an external bright light source, such as the sun, is reflected from the cathode. In this case, the reflected light from the cathode dominates the magnitude of light produced by the OLED display thereby reducing the perceived contrast of the image generated by the OLED display.
Circular polarizers can be used to improve the contrast of the image generated by the OLED display. The circular polarizer is mounted with adhesive onto the viewing surface of the OLED (for example, the viewing surface may be the bottom of the OLED). The circular polarizer works by absorbing ambient light that reflects from the cathode before it reaches the viewer. One disadvantage of using circular polarizers is that about sixty percent of the light emitted by the OLED is absorbed by the circular polarizer and never reaches the viewer. This absorption results in the necessity to drive the OLED at higher brightness considerably decreasing its life and increasing its power consumption. The increased power consumption is detrimental for battery-operated devices. Another disadvantage is that the emitted light that reaches the viewer is linearly polarized by the components of the circular polarizer. Thus, if the viewer is wearing polarized sunglasses, the display will be illegible at certain orientations. A further disadvantage is the processing difficulty of adhering the circular polarizer to the viewing surface of the OLED. Another disadvantage is that the circular polarizer, typically a polymeric film, mounted onto the viewing surface presents additional failure points in the overall OLED module, particularly under high heat and humidity conditions.
Another approach to improving contrast is the “black cathode” approach. This approach uses thin film interference effects at the cathode to eliminate ambient light reflections. In this approach, one or more additional layers is deposited during OLED fabrication behind the cathode layer. These one ore more additional layers are engineered to cause destructive interference of light at the cathode thus suppressing reflections. By controlling the thickness of the one or more additional layers, the phase shift of the light reflected by the one of those additional layers is such that it cancels the ambient light that is reflected (i.e., the light reflected by one of the additional layers and the reflected ambient light have almost equal amplitudes but are 180 degrees out of phase). The disadvantage of this approach is that in implementing it, the processing is difficult as the thicknesses of the one or more additional layers have to be exact in order to achieve the destructive interference and the processing of the OLED involves the deposition of additional layers.
In addition to the need for the cathode mentioned earlier, there also exists a need for an alternative to enhancing the contrast of the image produced by an OLED while not absorbing the light produced by it and while being easy to implement.